Synthetic Aperture Radar (SAR) systems have been utilized for many years for various applications and are well known for providing high resolution. For instance, SAR systems are well known as for identifying objects out of deliberate or natural clutter to provide a variety of functions including: area mapping, surveillance, and target detection. Further, these systems can be used from the ground (i.e., ground based SAR systems) as well as from the air (airborne SAR systems) and exploit the motion of an aircraft or vehicle, simulating a large antenna by combining return radar data along the flight path. This simulation is called the synthetic aperture.
Generally, airborne SAR systems are typically side-looking radars which produce two-dimensional (2-D) images of the earth's surface that are perpendicular to the aircraft path of flight and located on one side of the aircraft. One dimension in the image is called range (i.e., cross track) and is a measurement of the “line-of sight” distance from the radar to the target. Range measurements are determined by measuring the time from transmission of a pulse to receiving the echo from a target. Additionally, range resolution is determined by the transmitted pulse width. Cross-range resolution is achieved by coherently integrating the radar return signals along the flight path.
An example of a SAR system is disclosed in U.S. Pat. No. 7,796,829 (hereby incorporated by reference) entitled “Method and system for forming an image with enhanced contrast and/or reduced noise,” listing as inventors the same coinventors as the present application and filed on Dec. 10, 2008, and being assigned to The United States of America as represented by the Secretary of the Army.
To study the capabilities and limitations of utilizing the ultra-wideband (UWB) radar technology for the detection of concealed targets, the Army Research Laboratory (ARL) implemented different versions of UWB low-frequency SAR. A version of the UWB SAR radar that ARL designed and built is the vehicle-based radar which is discussed in detail in the publication by Marc Ressler, Lam Nguyen, Francois Koenig, David Wong, and Gregory Smith, entitled “The Army Research Laboratory (ARL) Synchronous Impulse Reconstruction (SIRE) Forward-Looking Radar,” Proceedings of SPIE, Unmanned Systems Technology IX, Vol. 6561, May 2007, which is hereby incorporated herein by reference. It is to be appreciated that the radar can be configured in both forward-looking and side-looking SAR modes.
Conventional side-looking SAR systems are designed such that the radar is typically mounted on an airborne vehicle (e.g., aircraft) whereby the transmitting and receiving antennas typically face the direction perpendicular to the flight path. Through signal processing, the reflected radar signals along the flight path are combined to form the SAR image for the area that exists along only one side of the reflected image.
FIG. 4 shows an illustration of a conventional side-looking SAR system used to survey a long strip of an area of interest 410. The challenge here is the detection of directional targets 1,2,3,4, which only have high reflectivity in a particular direction. Arrows extend from each directional target 1,2,3,4 representing the direction of high reflectivity or the target's individual radar cross section. Typically, the reflected signals from targets 1, 2, 3, 4 backscatter in directions that the radar will not be able to receive. In particular, an airborne vehicle 420, as shown in FIG. 4, passing over target 1 may only be able to recognize the target if it receives the return radar signals in the direction of the arrows extending therefrom. Similarly, the same is true for targets 2, 3, and 4. Consequently, in order to receive or detect the reflected signals being radiated from the targets 1,2,3,4, the airborne vehicle with radar platform must fly in a direction perpendicular to the area of interest 410, or alternatively, fly in a circular path around the area of interest 410 in order to capture all of the signals reflected from the targets. Thus, the overall operation is very impractical as it takes a long time to traverse all the pertinent paths to obtain target information regarding area of interest 410. This requirement significantly slows down the total SAR operation and further makes a surveillance task using these conventional systems impractical.
Additionally, FIG. 5 shows an illustration of a conventional forward-looking SAR system mounted to an airborne vehicle 520 used to survey a long strip of an area of interest 510. The radar might be able to receive return signals from some small sections of targets 1 and 3 along the flight path, but may not capture the return signals from targets 2 and 4.
As such, an ultra-wideband radar system is needed that can radiate and receive energy in all directions, thereby employing an array of antennas having the capability to capture the return radar signals in all directions.